This invention relates to a process for the digital to analog conversion of digital information in bipolar analog signals by means of a network comprising capacitors which are binary-stepped in their capacitance, and whose lower terminals referring to the drawing, depending upon the binary value of the digital information bit to which said lower terminals are allocated, are switched to ground potential or to a reference voltage source and thus either codetermine or do not codetermine the value of the network output voltage representing the analog signal after a charge has been redistributed.
When digital to analog converters (D/A converters) are used as a constituent part of interactive analog to digital converters (A/D converters) which must have a high resolution, the converter characteristic will require a high degree of linearity.
The same applies when D/A converters are a constituent part of an echo compensator which, in conjunction with the data transmission between subscriber station and telephone exchange of a communications system, is used via a two-wire circuit during full duplex operation. That is to say, the quality of echo cancellation depends to a large extent upon the non-linearities of the converter characteristic.
With a known D/A converter of the abovementioned type, which operates with a network of binary-weighted capacitors and is a constituent part of an A/D converter (High Speed CMOS A/D Conversion Techniques Techniques by William C. Black, pp. 44-53, 248-249, Memorandum No. Ucb/Erl MBO/54, November 1980), bipolar signals can be generated by means of a positive and a negative reference voltage (refer to FIG. 3.17 of the above Memorandum). When said reference voltages are not exactly equal, non-linearities are produced of an even-numbered order, the prevention of which requires alignment.
With another D/A converter of the kind referred to in the above Memorandum, by means of which bipolar signals are generated, only a signal reference voltage is required, while, on the other hand, the capacitor network has a capacitor with a peak-value capacitance which, when generating analog signals of one polarity, displaces the converter characteristic (refer to FIG. 3.19 of the above Memorandum) so that varying converter signals are required for the generation of positive and negative signals so that a non-linearity of an even-numbered order likewise exists.
Another known D/A converter for bipolar signals is used as a constituent part of an echo compensator and, likewise uses only one reference voltage, but, on the other hand, has an additional capacitor producing the aforementioned effects on the linearity of the converter. Steps can be taken for this D/A converter to reduce the influence of non-linearities on the results of echo cancellation in that, during the conversion process, one of the capacitors of the capacitor network, notwithstanding the process involving the remaining capacitors, is not connected directly to the reference voltage. The said reference voltage is fed instead to said capacitor via a voltage divider provided with non-linear tappings (Large Scale Integration of Hybrid Method Digital Subscriber Loops by Oscar Ernesto Agazzi, pp. 161-171, Memorandum No. Ucb/Erl M82/41, May 20, 1982). Since the tolerances of said additional capacitor, which essentially cause the non-linearities of the converter, are predominantly unsystematic, the elimination of the influence of said tolerances on echo cancellation by said means can be achieved only at disproportionate cost.